Add Custom Submenu to Add Menu
There are two changes necessary to add your custom menu entry.


menu_func has to be modified to add OBJECT_MT_effector_submenu to the menu
Don't call bpy.ops.wm.call_menu in register


The fixed menu_func looks like this:

def menu_func(self, context):
    self.layout.menu("OBJECT_MT_effector_submenu", text="Effector")